The present invention relates to a low thermal conductivity magnesite-spinel refractory shape and particularly brick suitable for use in cement kiln operations.
Magnesite-spinel brick are now commonly used in rotary cement kilns and such use has increased in recent years since the federal government has classified soluble chrome as a hazardous material. For many years prior to that time, magnesite-chrome brick provided excellent service and were extensively used throughout the rotary kiln industry. However it was found that chrome in such brick can react with alkali in the rotary kiln environment to form a soluble form of chrome known as hexavalent chrome. Such chrome was a potentially hazardous material and, thus, a used magnesite-chrome lining was classified accordingly and had to be disposed of as a potentially hazardous material which placed a large financial burden on operators of such kilns.
As a consequence chrome-free brick and in particular magnesite-spinel brick have been attempted to be used in place of such magnesite-chrome brick. However, while these brick have performed somewhat satisfactorily in service, they do not give a performance that matches that available with the magnesite-chrome compositions which replaced. One reason for this is due to the high thermal conductivity of the magnesite-spinel brick. Because of the bricks'higher thermal conductivity they do not insulate as well and therefore heat loss is greater and the brick are less economical.
In most extreme cases, hot spots develop on the outside shell of the kiln from extensive heat loss. As a consequence if such are not eliminated, it can soften the outside shell resulting in structural damage and costly repair. Consequently, the magnesite-spinel brick must be removed in such instances even though it has been found it still retains a relatively high percentage of its original thickness. In other words, the magnesite-spinel refractory lining has not undergone severe chemical or physical erosion, but, rather, the brick are replaced only because their thickness does not allow proper insulation of the outer metallic shell.
Efforts to overcome this problem in order to increase the useful life of the magnesite-spinel brick in these circumstances have not proven satisfactory.